Many analysis applications, such as chromatography and spectrometry, rely on the flow of fluid through one or more tubes and couplings. For example, in gas chromatography, one or more tubes, referred to as columns, are used to analyze a sample of material to determine its contents. In some applications, a first column is coupled to a second column, and, in other applications, a single column may be coupled to one or more restrictor devices, which may be coupled to various types of detectors. Regardless of the system topology, it is desirable to control the flow through the column or columns so that accurate analysis can be performed. The control of fluid flow is particularly important when performing multi-dimensional analysis, in which two or more columns (in the case of multi-dimensional chromatography) or two or more detectors (in the case of multi-dimensional detection) are used to perform the analysis.
The precise measurement and control of low flow rates, on the order of one milliliter per minute (mL/min), is difficult, costly and generally problematic. The measurement of pressure is often easier, less costly and more accurate, especially in systems with multiple flow paths and/or discontinuous physical dimensions or environment. Flow rates can be accurately deduced for open tubular flow paths using known relationships among pressures, physical dimension of the open tubes, temperature, and physical parameters of the type of gas flowing through the tubes.
During analysis, one or more variables, such as the temperature of the column in the case of chromatography, are frequently changed to perform the analysis. Further, in chromatography it is often necessary to change a column after some number of uses or to remove a contaminated portion of the column and reconnect it to the analysis device. Changing the temperature during analysis or altering the dimensions of the column alters the flow characteristics through the column, and necessitates the recomputation of various parameters so that the flow through the column may be accurately inferred and then controlled by adjusting pressure(s) to obtain the desired result. The inability to control the flow characteristics with multiple coupled tubes in an automated fashion in the current state of the art limits the utility and acceptance of multidimensional analysis techniques.
Therefore, it would be desirable to have the ability to automatically alter the flow in a chromatographic column, or a plurality of other open tubes, during analysis and to compensate for physical changes to the analysis systems.